1. Field of the Invention
The present invention relates to a spark plug for use in an internal combustion engine.
2. Description of the Related Art
Ordinarily, in spark plugs, many proposals for enhancing resistance to wear due to sparks, in which a noble metal chip mainly made of Pt, Ir or the like is welded on the front end of an electrode to form an wear resistant portion have been made. Particularly, the wear resistant portion on the part of a center electrode which, in many cases, is set to be negative polarity at the time of discharging a spark is liable to be subjected to a strong attack of the spark and be worn down, so that a noble metal has increasingly been used for this portion.
On the other hand, in regard to an internal combustion engine, a lean-burn method has increasingly been adopted as seen in direct fuel injection engines, to comply with strict exhaust gas control regulations. Also, in order to obtain optimum combustion, a configuration in which the spark discharge gap of the spark plug protrudes further inside a combustion chamber than an ordinary spark plug has increasingly been adopted. As a result, the electrodes of the spark plug, in particular the ground electrode which is located further inside the combustion chamber, is subjected to severe high temperature conditions. Therefore, it is an important task to improve heat resistance and spark wear resistance of the ground electrode and, accordingly, not only formation of a noble metal spark emitting portion but also changing the material of the electrode itself to a metal that has a higher heat resistance has been attempted. For example, Inconel 600 (trade name; available from Inco Ltd., in the U.K.), a Ni-based heat resistant alloy, has ordinarily been adopted as the material for the ground electrode, but adoption of Inconel 601 which has higher proportions of Cr and Fe and to which Al is also added to have a further enhanced high temperature strength and high temperature oxidation resistance is under consideration.
Heretofore, there are many cases in which a noble metal wear resistant portion of a ground electrode is formed by joining a noble metal chip to the ground electrode by means of resistance welding. However, the present inventors"" studies have found that, when the noble metal chip having a high melting point was resistance welded to a heat resistant alloy of higher grade such as Inconel 601 or the like, it was difficult to sufficiently ensure joining strength by resistance welding under the above-described severe use environment. Specifically, when the noble metal wear resistant portion is subjected to a severe heat cycle while the spark plug is in actual use, the noble metal wear resistant portion is peeled away from the ground electrode so that normal ignition can not be performed.
Heat resistant alloys such as Inconel 601, in which the proportion of corrosion resistance-improving components such as Cr, Al or the like are increased, intrinsically have the tendency to decrease in weldability in direct proportion to an increase in oxidation resistance. Therefore, it may seem at first that the cause of the above-described decrease of joining strength is insufficient melting due to such a decrease of weldability, but the present inventors"" research has found that the intrinsic cause is not insufficient melting.
It is therefore an object of the present invention to provide a spark plug in which, even when the ground electrode comprises a heat resistant alloy having an increased content of corrosion resistance improving component such as Cr, Al, the peeling resistance of a noble metal wear resistant portion welded to the ground electrode can sufficiently be ensured, whereupon the spark plug can be used for a long period of time even under severer working conditions.
A spark plug according to the present invention, in which a spark discharge gap formed by securing a noble metal wear resistant portion to the face of a ground electrode facing the front face of the center electrode, is characterized in that a portion containing at least the sides of the ground electrode comprises a Ni alloy containing from 21% by mass to 25% by mass of Cr, from 1% by mass to 2% by mass of Al, from 7% by mass to 20% by mass of Fe and from 58% by mass to 71% by mass of Ni, the noble metal wear resistant portion is joined to the side face of the ground electrode via a welding portion, xcex94xcex1xe2x89xa1xcex12xe2x88x92xcex11 is adjusted to be 4.55xc3x9710xe2x88x926/K or less, wherein xcex11 represents the linear expansion coefficient at 800 K of the noble metal constituting the noble metal wear resistant portion; and xcex12 represents the linear expansion coefficient of the Ni alloy constituting a portion comprising the above-described side face of the ground electrode, and, further, the outer diameter of the noble metal wear resistant portion, which is defined as the diameter of the circle having the same surface area as that of an orthogonal projection of the noble metal wear resistant portion on a plane perpendicular to a center axis line of the center electrode, is from 0.6 mm to 1.5 mm.
In the above configuration, a raw material (hereinafter referred to as an electrode base material) composing a side face portion of the ground electrode which is subjected to high temperature particularly while the spark plug is in actual use is made to be a Ni alloy having the above-described composition which is excellent in high-temperature resistance and anti-oxidation property to a further extent than Inconel 600 or the like which has ordinarily been used. As a result, durability of the spark plug according to the present invention in a high temperature environment is enhanced and defects thereof such as corrosion, breakage or the like are less liable to occur.
Further, the present inventors have studied in detail factors affecting peeling resistance in the case where the noble metal wear resistant portion is joined to the electrode base material having the above composition, and it was clearly found that peeling is caused by a difference in the linear expansion coefficients of the noble metal constituting the wear resistant portion and the electrode base material rather than by insufficient melting due to decreased weldability of the electrode base material or the like. Accordingly, as a result of still further study it has been found that the peeling resistance of the noble metal wear resistant portion on the ground electrode can be enhanced to a great extent by adjusting xcex94xcex1xe2x89xa1xcex12xe2x88x92xcex11 to be 4.55xc3x9710xe2x88x926/K or less, wherein xcex11 represents the linear expansion coefficient at 800 K of the noble metal constituting the noble metal wear resistant portion; and xcex12 represents the linear expansion coefficient at 800 K of the electrode base material.
However, it has been found that, under a use environment in which attainable temperature of the ground electrode is higher than ordinary, for example, during high speed or high load driving with a lean-burn or direct fuel injection engine, it is difficult to sufficiently ensure peeling resistance of the noble metal wear resistant portion only by adjusting such a difference xcex94xcex1 in linear expansion coefficients. To deal with this, the outer diameter of the noble metal wear resistant portion on the ground electrode as defined above is made to be from 0.6 mm to 1.5 mm, by which the peeling resistance can further be enhanced and the durability of the noble metal wear resistant portion can sufficiently be ensured even under such a severe use environment as described above.
As for the composition of the Ni alloy constituting the electrode base material, the Cr content can be from 21% by mass to 25% by mass. When the Cr content is less than 21% by mass, it is difficult to ensure desired high temperature oxidation resistance and high temperature strength. On the other hand, when the Cr content is over 25% by mass, ductility of the material is deteriorated whereupon impact resistance is deteriorated and, simultaneously, workability is deteriorated, causing an increase in production cost.
Further, Fe content can be from 7% by mass to 20% by mass. When the Fe content is less than 7% by mass, it is difficult to ensure desired high temperature strength. On the other hand, when the Fe content is over 20% by mass, the ductility of the material is deteriorated whereupon impact resistance is deteriorated and, simultaneously, workability is deteriorated, causing an increase in production cost.
Further, Al content can be from 1% by mass to 2% by mass. When the Al content is less than 1% by mass, it is difficult to ensure desired high temperature oxidation resistance. On the other hand, when the Al content is over 2% by mass, the ductility of the material is deteriorated due to formation of an intermetallic compound such as Ni3Al or the like so that impact resistance is deteriorated and, simultaneously, workability is deteriorated, causing an increase in production cost.
Ni is the main element of the above substances, constituting the portion remaining after the above-described auxiliary elements are excluded. Here, when Ni content is less than 58% by mass, it is difficult to ensure the desired high temperature oxidation resistance. On the other hand, in view of respective minimum quantities of auxiliary elements, the Ni content can not be over 71% by mass.
As a Ni alloy having the above-described composition, Inconel 601 can be used. The standard composition thereof is Ni: 60.5% by mass, Cr: 23% by mass, Al: 1.5% by mass, Fe: 14.1% by mass, Mn: 0.5% by mass, Si: 0.2% by mass and C: 0.05% by mass.
Further, in the case where xcex94xcex1 is over 5.7xc3x9710xe2x88x926/K, when a severe heat cycle is applied, sufficient peeling resistance of the noble metal wear resistant portion can not be ensured. The linear expansion coefficient xcex12at 800 K of the electrode base material having an alloy composition in the above-described range is ordinarily confined in a range of from 15.2xc3x9710xe2x88x926/K to 15.4xc3x9710xe2x88x926/K (for example, 15.3xc3x9710xe2x88x926/K in the case of Inconel 601). On the other hand, although the linear expansion coefficient xcex11 at 800 K of the noble metal wear resistant portion is smaller than the linear expansion coefficient xcex12 of the electrode base material, the value of xcex11 varies to a great extent depending on noble metal compositions. Therefore, taking the value of xcex12 of the selected electrode base material (Ni alloy) into consideration, the noble metal composition to be used is selected so as to have a linear expansion coefficient xcex11 which is as close as possible to the value of xcex12.
The noble metal wear resistant portion may be made with any type of metal element as a main component so long as it is a noble metal (that is, having a noble metal content therein of 50% by mass or more) and, also, among metal elements ordinarily called noble metals, any metal element having a relatively high melting point (such as Pt, Ir, Rh, or Ru). Further, xcex94xcex1 can be 0; however, when only the noble metal compositions which can sufficiently ensure wear resistance are considered, it is difficult in practice to have a xcex94xcex1 value of 0.8xc3x9710xe2x88x926/K or less.
Not only in view of the need that xcex12 be brought near xcex11, but also in view of the need that the wear resistance be ensured, it is preferable that the noble metal wear resistant portion has Pt as its main component. In this case, the linear expansion coefficient at 800 K of Pt is 10.3xc3x9710xe2x88x926/K. When the noble metal wear resistant portion has Pt as its main component, in order to further enhance wear resistance at a high temperature, Ptxe2x80x94Pdxe2x80x94Ru alloy which contains Pd or Ru can also be adopted. In this case, as the content of Ru becomes lower, xcex11 becomes smaller (that is, xcex94xcex1 becomes larger) whereupon it is necessary to select the content of Ru within a range in which xcex94xcex1 is not over 4.55xc3x9710xe2x88x926/K. Further, if xcex12 is to be brought to near xcex11, employing a Ptxe2x80x94Ni alloy in which Ni, which exhibits a remarkable action in increasing the linear expansion coefficient, is added to Pt is also effective (for example Pt:Ni is 20% by mass).
Next, the outer diameter of the noble metal wear resistant portion on the ground electrode is, as described above, made to be from 0.6 mm to 1.5 mm. When the outer diameter of the noble metal wear resistant portion is over 1.5 mm, it is difficult to ensure the desired peeling resistance. This is attributable to the increase in the joint interface area of the electrode base material and the noble metal wear resistant portion, generating large displacement along the joint interface by heat expansion/contraction at the time of heating/cooling, whereupon peeling is liable to occur. Thus, by making the above-described outer diameter of the noble metal wear resistant portion 1.5 mm or less, the peeling resistance can further be enhanced whereupon durability of the noble metal wear resistant portion can be ensured even in such a severe use environment as described above. On the other hand, when the outer diameter of the noble metal wear resistant portion is less than 0.6 mm, sufficiently long life of the noble metal wear resistant portion can not be ensured. Further, in the present invention, the outer diameter of the noble metal wear resistant portion is defined by an outer diameter of an orthogonal projection of the noble metal wear resistant portion on a plane perpendicular to a center axis line of the center electrode. Furthermore, in the present invention, the shape of the above-described orthogonal projection of the noble metal wear resistant portion can be a circle but is not limited thereto and can have corners instead.
Additionally, it is preferable that, in order to further enhance peeling resistance of the noble metal wear resistant portion, the size of the noble metal wear resistant portion is adjusted such that S/T is from 0.7 mm to 4.5 mm, wherein T represents thickness of the noble metal wear resistant portion; and S represents the projection area of the noble metal wear resistant portion on a plane perpendicular to the center axis line of the center electrode. When S/T is less than 0.7 mm, the thickness of the noble metal wear resistant portion becomes relatively unduly large and, as a result, when a cooling/heating cycle is applied, stress acting on a joint interface between the electrode base material and the noble metal wear resistant portion becomes large, unfavorably affecting peeling resistance. On the other hand, when S/T is over 4.5 mm, the thickness of the noble metal wear resistant portion becomes unduly small and, as a result, there are some cases in which sufficiently long life of the noble metal wear resistant portion cannot be ensured.